Solving power-quality problems

Electronic equipment, which is now so integral to industrial and commercial power systems, can fail or malfunction if subjected to a voltage, current, or frequency deviation.Before the era of solid-state electronics, power quality was not discussed because it had little or no effect on most loads connected to electrical distribution systems. When an induction motor suffered a voltage sag, it didn't

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Electronic equipment, which is now so integral to industrial and commercial power systems, can fail or malfunction if subjected to a voltage, current, or frequency deviation.

Before the era of solid-state electronics, power quality was not discussed because it had little or no effect on most loads connected to electrical distribution systems. When an induction motor suffered a voltage sag, it didn't shut itself down but simply "spun out" fewer horsepower until the sag ended. The same was true for incandescent or fluorescent lighting systems in a facility-the lumen output just decreased temporarily.

But today, as sensitive equipment and processes become more complex and downtime costs increase, contractors and engineers have to specify and install specialized equipment to fight off danger.

The ideal power-supply voltage for sensitive electronic equipment is an uninterrupted sinusoidal waveform of constant amplitude. Any event that reduces this condition is called a disturbance. Disturbances as brief as one-half cycle can affect the operation of sensitive electronic equipment.

Defining the disturbances Voltage dips or sags are usually caused by poorly regulated utility feeders that have to deal with significant load changes over time. Voltage swells, which are brief increases in the root mean square (RMS) voltage, often accompany voltage sags. They can appear on unfaulted phases of the three-phase circuit with a single-phase short circuit. A swell can upset electronic controls, such as an adjustable-speed motor drive, which can trip because of its internal protective circuitry. Such an overvoltage can also degrade insulation materials and thus shorten the life of electrical equipment.

An undervoltage, due to a deliberate utility reduction or an incorrect transformer setting, might lower the kVAr output of a capacitor bank installed to help maintain voltage and power factor in an industrial plant with large inductive loads (motors).

Transients are voltage disturbances of a much shorter duration than sags or swells. They fall into two classes: impulsive transients-often caused by lightning strikes and utility load switching-and oscillatory transients, which are usually caused by capacitor bank switching. A utility capacitor bank is usually switched to keep up with changes in power demand. Except for lightning occurrences, almost all transients are created by the interaction between stored energy in circuit inductances and capacitances.

A condition known as current chopping can occur in a facility when current is interrupted at peak flow or when a light load, such as the excitation current of an unloaded transformer, is interrupted. The severity of such a condition increases with the speed of the interruption, such as when current is interrupted by a current-limiting fuse, a thyristor, an SCR, or a mercury arc rectifier. The first line of defense against various transients is the transient voltage surge suppressor.

A systematic approach Complaints from facility site personnel often starts the investigation of an electrical power problem.

At the outset, the investigator (contractor) has to determine whether the problem originates onsite, or whether it comes from the utility service. This is where a three-step procedure is useful:

1. Refer to the power system's one line diagram and to other important data to gain an initial knowledge of the power system layout.

2. Inspect the power system wiring (feeders, branch circuits, etc.) for any defective connections or wiring errors.

3. Conduct field measurements to uncover the source, or sources, of the disturbance. These field measurements are useful because two or more events occurring together often cause the problem.

Inspecting power-system wiring and system grounding is extremely important because studies show that incorrect wiring and grounding errors cause from 50% to 80% of power-quality problems.

It also includes any supplementary connections between the above mentioned items and structural building steel, earth grounding electrodes, and similar items.

The equipment grounding system must conduct ground-fault current. And at the same time, it must keep touch potentials under control so personnel do not experience any significant shock hazards.

Unfortunately, equipment grounding designs at Information Technology Equipment (ITE) installations often don't meet one, or both, of these requirements because either the original equipment manufacturer (OEM) or the electrical system's designer or installer has replaced NEC requirements with personal ideas.

NEC-compliant equipment grounding is fairly simple to achieve. The feeders and branch circuits must use either a suitable metal conduit/raceway, an equipment (safety) grounding conductor, or both, for equipment grounding. As the NEC requires, the equipment grounding conductor (or "green wire" as it is called) has to run inside the same conduit/raceway as the circuit conductors or it won't work efficiently. For electronic equipment, the institute of Electrical and Electronic Engineers (IEEE) recommends the use of both a metal conduit/raceway and a green conductor (wire) with each circuit. All terminations, connectors and fittings must be tightly installed on the conductor/raceway. Loose joints and terminations can cause trouble for a number of reasons.

Performance grounding, involving the use of grounding reference for equipment, ac systems and signal processing, uses a signal reference structure (in the form of grid or plane) as described in the IEEE Emerald Book. A recent addition to the IEEE color book series, IEEE Standard 1100 Recommended Practice for Powering and Grounding Sensitive Electronic Equipment is a good reference.

If problems still continue after wiring is checked and corrected, then voltage and load current measurements should be made. Today, a wide variety of instruments can provide both graphic and alphanumeric readouts. A typical modern portable test instrument for three-phase power analysis can measure, record, and analyze volts, amps, watts, VA, VAR, power factor, crest factor, total harmonic distortion, and demand. Waveforms and other power-quality disturbances can be viewed with event and worst-case reports.

Transient-recording instruments must have a frequency response of 100 kHz or higher to be useful. They also need special triggering techniques, such as post triggering, to capture data. Several voltages and currents might have to be monitored or triggered simultaneously to relate to switching events so that qualitative analysis of the readings can be done. In some cases, an engineering consultant specializing in power quality analysis may be needed.

Understanding harmonic distortion A pure sinusoidal voltage doesn't really exist in today's power system where loads classified as nonlinear loads are used. Examples are electronic ballasts, variable frequency drives and switching mode power supplies in personal computers. These equipment types cause current and voltage waveforms that are nonsinusoidal (they have distortions), resulting in additional waveforms being superimposed on the standard 60-Hz waveform. The additional waveforms are harmonics of the fundamental frequency. Each full integer multiple of the frequency is called an order.

The biggest effect of harmonics is the increased heating of a power transformer's core. The degree of increased heating depends on a few variables: the transformer's construction, its load and the magnitude of harmonic distortion. Remedies for the presence of harmonics include redistributing loads, derating an existing transformer or installing new K-rated (or K-factor) transformers.

A transformer's K-factor rating describes how well it can handle the additional heat generated by high harmonic content. You can find out more in the ANSI/IEEE Standard C57, 110-1958 Recommended Practice for Establishing Transformer Capability When Supplying Non-Sinusoidal Load Currents.

Looking at equipment Perhaps two of the most widely used power conditioning devices used today is the transient suppressor and the isolation transformer. The lighting arrester is typically used outdoors on utility distribution systems, etc., to shunt perhaps thousands of amperes of current to ground and sacrifice itself. Essentially, the lightning arrester allows the lower-power-level transient suppressors installed within a facility to handle the remaining energy of the transient impulse.

In shunting the destructive electrical energy to ground, the transient suppressor can create an undesirable situation. It can change a transverse-mode problem (noise on power conductors feeding the load) into a common mode problem, with noise voltage appearing equally and in-phase from each current-carrying conductor to ground. Common mode noise may cause control system shutdowns because of this corruption of signal transmission.

A shielded isolation transformer is able to provide good common mode noise isolation. It can have a 1-to-1 ratio for isolation only, or it can provide voltage transformation. The minimum shielding is usually a grounded Faraday shield between windings, but additional attenuation can be gained by also shielding either or both individual windings.

A voltage regulator is any device that regulates output voltage to a set value with any variation in input voltage. This description covers constant voltage transformers, power enhancement devices and power synthesizers. Selecting the best product requires careful study because each technology has its own benefits.

At locations where electronic process controllers, critical sequence-timed equipment for manufacturing and warehousing, and at data processing sites, a battery-supported UPS is the answer to a complete power outage. Since battery capacity is limited, an on-site standby generator is needed to support operations through an extended outage.

An alternative to the battery-supported static UPS system is the rotary UPS system. Such rotating units use a flywheel principle to override the power interruption until a generator can be started. In one typical 200 kVA unit, the flywheel resides in a cabinet containing the spinning steel flywheel, which rides in a vacuum chamber on hybrid mechanical and magnetic bearings. Its kinetic energy provides continuous dc power for up to several minutes, depending on load characteristics when power falters. The rotary UPS' return-on-investment is enhanced; fewer batteries are required and fewer charging and discharging cycles extend battery life.

Learn about your markets Contractors wanting to expand into power-quality work can team with telecommunications consultants specializing in phone and data networking installations. For example, one insurance industry telecom-consulting firm recommends power protection as part of its service offering. This reseller supplies its customer with a turnkey system: software, PCs, servers, hubs, and routers. But hardware and software is only part of the reseller's value proposition; the other is to keep the mission-critical systems up and running. Thus, the consultant recommends an "intelligent" uninterruptible power-supply unit (UPS) for each server. The specifically recommended UPS is a line-interactive design that keeps a proper power level flowing to the server through under-and over-voltage conditions. This particular UPS unit can also supply log reports that facilities technicians can use to spot power problems. For example, one business in the area was altered to a power problem by the "intelligent UPS." It was signaling a constant fluctuation in the voltage of the circuit powering its server. A local electrical contractor found that the circuits were overloaded and needed rewiring. If the UPS had not been installed, the problem probably would not have been detected.

As added insurance, the consultant also recommends customers buy software that works in conjunction with the UPS to shut down a protected server automatically during a power failure. Without it, data can be lost and applications and databases fouled, requiring several hours of downtime to run repair utilities.

The consultant notes that most customers don't appreciate the importance of power protection. One customer made the fateful decision to leave its network completely unprotected without a UPS system or even a surge suppressor. A power surge traveled along the ac line, destroying its server and concentrator.

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